1. Field of the Invention
The present invention relates to purge and trap concentrator systems, and, more particularly, to those that include a sparge vessel. The invention, in various embodiments, includes: (1) a variable gas flow valve for controlling the gas pressure in the sparge vessel, or in an analytic trap; (2) a foam sensor subassembly that detects both a foaming sample state and a high liquid level in the sparge vessel, using a single optical sensor; (3) a control scheme in which a purge and sampling procedure can continue after a foaming sample has been detected, by re-directing the purge gases to a second inlet of the sparge vessel; (4) a control scheme in which a desorbtion mode uses a split flow to enhance the quantity of sample gases that are passed to an analyzer instrument from an analytic trap; (5) a thermal heater subassembly that uses an electrically powered thermal energy source and a fan to raise the temperature of the sparge vessel via thermal convection during a bake mode, by directing heated air to the sparge vessel using a ductwork arrangement with the fan; and (6) a desorbtion pressure control mode in which the pressure at an analytic trap is brought to a controlled value that will allow the system to jump to the inject mode without undergoing a pressure surge at the trap.
2. Description of the Related Art
Purge and trap concentrators have been is used to extract VOCs from aqueous samples, or from a solid sample matrix. In many systems, a sample is housed in a sealed vessel known as a sparge vessel. The sparge vessel typically is constructed in a U-shape design with an inlet side (purge portion) and an outlet side (sample portion). An inert gas (purge gas) of helium or nitrogen sweeps the aqueous sample at a controlled flow rate known as purging. The purge gas is introduced on the purge portion of the sparge vessel; typically the purge gas stream passes through a frit placed in the bottom of the sample portion of the sparge vessel. The frit disperses the gas into many fine streams to increase surface contact of the gas with the sample for extraction of VOCs. One problem with purging aqueous samples is they have a tendency to foam, and if the foam is left unattended the sample could come in contact with internal pathway components of the instruments pathway causing contamination to the entire system. This can require costly repairs and downtime for the testing laboratory, along with rendering the analytical test data invalid.
Today most purge and trap concentrators have a foam detection sensor that will turn off the purge gas and stop the sampling process, or continue in a “safe mode” to prevent costly repairs. The problem here is that the sample is then wasted and deemed unusable. The laboratory will have to re-run the sample (if it has one) or contact the client to send a new sample.
One approach to this problem is pre-treating suspect samples with an anti-foaming agent, such as Dow Corning Silicone RID emulsion. However this treatment can raise concerns regarding the sample integrity. A second approach is to disrupt the foam with a heat source to continue the analysis. However the heating of the foam and headspace of the sample containing extracted VOCs could also raise some questions on the integrity of the extracted VOCs.
Most purge and trap concentrators using a sparge vessel provide an inert (purge) gas that is swept through the concentrated chemical sample at a controlled flow rate, thereby extracting the VOCs from the sample. The extracted VOCs are then placed in fluidic communication with an adsorbent trap for concentrating. The adsorbent trap is thermally heated to release the extracted VOCs from the adsorbent bed of the trap. A second passage of inert (carrier) gas is in fluidic communication with the adsorbent trap to back-flush the VOCs from the adsorbent trap to an analytical device know as a Gas Chromatograph (GC) for separation and identification.
Conventional purge and trap concentrators typically contain a switching device for the purpose of networking the fluidic communication of purge gas and carrier gas to the adsorbent trap during the desorbtion step. The gas chromatograph typically controls the carrier gas flow rate, as the purge and trap concentrator controls the purge gas flow rate. The pressure and flow rates for the purge and carrier gas usually differ, causing some dead volume issues during the switching of fluidic communication of purge gas pathway to the carrier gas pathway during the desorbtion step. The dead volume can affect the transfer rate and analytical resolution of the extracted VOCs. When sampling an aqueous sample matrix, depending on the carrier gas flow settings and purge and trap desorbtion settings, an unwanted amount of moisture content is sometimes transferred to the gas chromatograph, which affects analytical resolution and detected recovery of the extracted VOCs.
Conventional purge and trap concentrators using a sparge vessel are often used to extract VOCs, typically from an aqueous or a solid sample matrix. The purge and trap concentrators typically consist of three stages for a completed sample analysis cycle: (1) a purge step, (2) a trap desorbtion step, and (3) a bake step. The bake step is a system “cleanup” step used for preparing the system to receive consecutive concentrated samples for analysis. Typically a gas flow is in fluidic communication with the adsorbent trap and the sparge vessel for the purpose of preparing the sample pathway for the next sample analysis. This gas flow typically sweeps the entire sample pathway, while concurrently thermally heating the adsorbent trap to a setpoint temperature higher than the desorbtion setpoint temperature, for the purpose of removing contaminates from the trap in preparation of next sample analysis. If the purge and trap concentrator is connected to a vial auto sampler, such as an EST Analytical Centurion™ Vial Auto Sampler, a heated rinsing liquid (typically deionized water) is flushed through the sparge vessel for cleaning the sparge vessel of unwanted contaminates, concurrent with the bake step. A low amount of carryover of unwanted contaminates sometimes will exist, even in today's purge and trap concentrators.
An improved purge and trap concentrator is needed to detect and prevent foaming without questioning the integrity of the extracted VOCs, to provide a method for removing the remaining contaminates from the sampling pathway that could affect the reporting of analytical data from the subsequent sample analysis, and a method for providing appropriate sampling process parameters to improve overall analytical analysis of extracted VOCs.